Various lead silicate glasses which absorb x-rays have long been used to construct vacuum tubes for television receiver sets being operated in the 20-40 KV region. For example, there is disclosed in U.S. Pat. No. 3,854,964, assigned to assignee of the present invention, a glass composition used in this manner consisting of, by weight, 30-40% SiO.sub.2, 3-7% K.sub.2 O, 0-2% Na.sub.2 O, 0.3% Al.sub.2 O.sub.3, 35-50% PbO, 0-15% BaO, 0-15% Sb.sub.2 O.sub.3 and 0-15% CaO. In still another now abandoned patent application, Ser. No. 100,407, also assigned to the present assignee, there is disclosed a different glass composition for the same purpose having as its essential constituents by weight, 40-48% SiO.sub.2, 3.5-5.5% Al.sub.2 O.sub.3, 0.8-2.8% Na.sub.2 O, 9-12% K.sub.2 O, 28-37% PbO, 0-3% CaO and 2-10% BaO. In said latter defined glass composition, it was found that adding BaO to this lead glass improved the x-ray absorption in the "hard" X-ray region of 30-40 KV. A still different glass composition now being commercially employed as the face plate component in this type vacuum tube is reported to have a composition in weight percent 59.6% SiO.sub.2, 3-6% Al.sub.2 O.sub.3, 13.6% combined BaO plus SrO content, 4.2% CaO, 1.7% MgO, 7.3 Na.sub.2 O, 8.0% K.sub.2 O, 0.6% F, 0.15% CeO.sub.2, 0.2% AS.sub.2 O.sub.3, 0.4% B.sub.2 O.sub.3, and 0.1% PbO.
As can be noted in the development of the aforementioned prior art glasses for the present product application, the removal of arsenic as a refining agent has taken place for ecological considerations. The replacement of arsenic with antimony as the principal refining agent results in certain drawbacks, however, both during melting of the glass itself wherein a higher seed count in the refined glass is experienced as well as incurring higher thermal shock breakage when the solid glass product is thereafter thermally reworked generally with gas oxygen flames when assembling the vacuum tube parts together. Both of said drawbacks are believed attributable to lowering the refining and oxidizing action when antimony oxide replaces arsenic oxide as the refining agent. A further addition of niter to the antimony refining system has also not alleviated the aforementioned drawbacks which may account for the continued use of arsenic as an essential refining agent when melting lead silicate glasses.